During melting and refining of steel, the gas leaving the furnace carries a substantial amount of fine dust particles. To protect the atmosphere, these particles are removed in cyclones, scrubbers, and other similar devices. The dust particles are made up of two types of materials. Firstly, it consists of fine particles charged to the furnace such as those of iron oxides, calcium oxide, and silica, which are mechanically carried away by the gas. Secondly, it consists of oxide particles of nonferrous metals such as zinc and lead, which evaporate at the high temperatures of the steel bath and later condense and oxidize in the flue. The amounts of these nonferrous metal oxides in the dust increase as the dust is repeatedly recycled or as more scrap containing these metals are charged into the furnace.
The dust can be repeatedly recycled only for a limited number of cycles. With each recycle, the amount of dust to be collected increases due to larger proportion of the very fine dust particles in the feed. The feeding of such fine particles of low bulk density becomes increasingly difficult. Furthermore, as the amount of recycled dust increases, the energy requirement for reducing and melting iron and other metals from the dust increases.
Also, with repeated recycling, the nonferrous metal content of the dust increases to the point where their return to the steel bath adversely affects the steel melting operation. As oxides present in the slag, they attack the refractory lining of the furnace. In reduced form, these metals enter the molten steel at concentrations higher than permissible to meet specifications for proper mechanical and physical properties of the steel. At this point, the dust has to be removed from the system. Traditionally, this dust has been considered a waste material and disposed of in landfills.
This disposal method is becoming more expensive because of increasing land and transportation costs and because of increasingly stringent environmental protection regulations. Since the dust particles are very fine, they are likely to become airborne unless they are agglomerated to some extent or kept covered. Furthermore, the chemical nature of these dust particles is such that they classify as hazardous waste, based on the EP toxicity test prescribed by the United States Environmental Protection Agency. As disclosed in "Characterization of Steelmaking Dusts from Electric Furnace", United States Bureau of Mines Report of Investigation No. 8750 (1983) by S. L. Law et al., the toxicity is related to the presence of lead, cadmium, and sometimes chromium.
It would, therefore, be desirable to provide a method that would remove nonferrous metals like zinc, lead, and cadmium from the dusts and convert the remaining iron-bearing residue into a physical form suitable for easy recycling to the steel furnaces or for storage.